With the progress in the digital technology of electronic devices in recent years, nonvolatile semiconductor memory devices with a large capacity have been actively developed for storing data, such as music, images, and information.
Furthermore, a variable resistance nonvolatile semiconductor memory device (hereinafter referred to as a variable resistance memory) using a variable resistance element in which a resistance value changes according to the application of an electrical pulse and the state is maintained draws attention in that it is easy to ensure consistency with a normal semiconductor manufacturing process.
The variable resistance memory includes two electrodes and a recording layer comprising a metal oxide (variable resistance layer) provided between the electrodes, reversibly changes the recording layer between a high resistance state and a low resistance state by applying an electrical pulse between the electrodes, and is used as a memory by associating different resistance values generated from the change with data “1” and data “0”, respectively.
As disclosed in Patent Literature 1, a variable resistance memory using a tantalum oxide thin film with nonstoichiometric composition which shows electrical conductivity in the variable resistance layer have good variable resistance characteristics in that the memory can operate at a fast speed of no more than 100 ns and can rewrite no less than 1000 times.
The tantalum oxide thin film with nonstoichiometric composition, using metal tantalum as a target, is formed by a reactive sputtering method using argon and oxygen gas as a sputtering gas.
Moreover, Patent Literature 2 discloses that by repeating a process of forming a metal oxide film by reactive sputtering using a metal target and a process of further oxidizing the surface of the metal oxide film with thermal processing, a laminate of a plurality of metal oxide films each of which is treated with surface oxidation is formed. Then by treating the laminate with thermal processing in the last stage, a metal oxide layer is formed with an intended resistivity from the laminate.